Multilayer, thermoplastic films are being used for packaging various food and non food products because they protect the item itself from the environment during storage and distribution. Furthermore, for the end consumer, it is desirable to present the product packaged in a preferably transparent thermoplastic film that allows visible inspection of the package contents to help assure the quality of the product.
Optical characteristics are therefore often important for a thermoplastic film for packaging.
Other properties are also desirable, such as good mechanical properties that keep the package unaltered until it is offered to the customer on consumer.
A shrink feature can also be imparted to a thermoplastic film by orientation or stretching of the film, either mono-axially or biaxially, during film manufacture. This shrink feature allows the film to shrink or, if restrained, create shrink tension within the film upon exposure to heat. In a typical process, the thick structure which is extruded through either a round or a flat extrusion die is quickly quenched, then it is heated to a suitable temperature, called the orientation temperature, which is higher than the glass transition temperature (T.sub.g) of the resins used in the film itself but lower than the melting temperature (T.sub.m) of at least one of the resins, and stretched in either or both of the machine (longitudinal) and transverse directions.
For food packaging, it is often necessary that the film has oxygen barrier characteristics to delay or avoid product oxidation or degradation during its shelf-life.
Good heat sealability is sometimes required. It is essential, particularly for oxygen barrier films used in applications where the contained product is to be kept either under vacuum or under a modified atmosphere, that the seal(s) that close the package have adequate strength and, as a consequence thereof, that the package remains tight.
Several different materials have been used to decrease the oxygen permeability of thermoplastic films. Among these materials a very good gas barrier material is EVOH (ethylene/vinyl alcohol copolymer). Several "barrier" thermoplastic films comprising an EVOH layer are described in the patent literature.
In prior art films the different materials employed for the skin layers are suitably combined with the aim of improving as much as possible the film characteristics, particularly those characteristics that are needed for the specific intended applications. As an example, the use of a low density ethylene/alpha olefin copolymer provides for fair heat-sealability and remarkable oil resistance properties; the use of EVA (ethylene/vinyl acetate copolymer) improves the shrinkability and the sealability properties; the use of propylene homo- and/or copolymer increases the stiffness of the structure; etc.
It is however known that a resin that improves a specific property typically worsens other properties, and therefore the research efforts in this field tend to reach an optimum balance of these properties.
More particularly the film characteristics that still need to be improved, in a way that however should not negatively affect the other characteristics such as optical, mechanical, barrier and shrinkability properties, are the sealability properties.
It has now been found that it is possible to provide a film with optical and gas barrier properties at least comparable to those of known films containing EVOH, and having remarkably improved mechanical and heat-sealability properties, by using in the film sealing layer a blend of ethylene/alpha olefin copolymers of suitably selected different densities.